The integrity of the adult skeleton undergoes constant and dynamic remodeling to maintain bone homeostasis, which is achieved by the tight control of coupling between osteoclast-mediated bone resorption and osteoblast-mediated bone formation. However, the balance of this bone remodeling process is disrupted in pathological conditions. Chronic inflammation is one of the most evident and common pathological settings that often lead to deregulated bone remodeling characterized by excessive bone loss but limited bone formation, such as occurs in rheumatoid arthritis (RA), periodontitis and peri- prosthetic loosening. The disturbances in bone remodeling in these diseases are refractory to current antiresorptive treatments and lead to continuous musculoskeletal tissue damage and accelerating pathogenesis. The mechanisms that impact the deregulated bone remodeling in pathological conditions are largely unknown, which is a barrier to successfully treating the numerous diseases associated with bone loss. Our long term goals are to identify and understand the mechanisms that regulate bone remodeling in inflammatory settings, and to utilize this knowledge in development of new therapeutic approaches to diseases associated with inflammatory osteolysis. We are particularly interested in TNF- ??mediated bone remodeling since TNF-? is a key pathogenic factor driving inflammatory bone resorption. We have recently identified Def6 that functions as a novel regulator of bone remodeling in both physiological conditions and TNF-?-mediated inflammatory settings. Def6 suppresses osteoclast differentiation while enhancing osteoblast differentiation in response to TNF-? stimulation. Importantly, Def6 expression levels are downregulated by TNF-? and inversely correlate with TNF-? levels in RA patients, indicating a potentially important role of Def6 in the pathogenesis of deregulated bone remodeling in RA. In this application, we will apply genetic approaches and use both in vitro cell culture systems and arthritis animal models to investigate: 1) the mechanisms by which Def6 regulates bone remodeling factors, with a focus on the epigenetic regulation of DKK1 expression and the DKK1-Wnt mediated remodeling axis by Def6; 2) the mechanisms by which Def6 regulates both osteoclast and osteoblast differentiation; and 3) the functional importance of Def6 in inflammatory arthritic bone resorption using both loss and gain of function of Def6 in inflammatory arthritis animal models. A structure-function analysis of Def6 will be performed to identify the domain(s) and/or phosphorylation site(s) that are important for Def6 function in bone remodeling. We anticipate that our studies will yield insight into mechanisms that restrain inflammatory osteolysis, and will be useful in developing new therapeutic approaches to suppressing bone resorption in inflammatory settings.